1. Field of the Invention
The present invention concerns a new method for the fabrication of ceramic material, chiefly with a perovksite structure, by hybrid chemical process, in using a mixture of mineral precursors and organic precursors.
2. Description of the Prior Art
To define a ceramic, we might refer to the article by J-M Haussonne in Techniques de l'Ingenieur, E 1820, p. 3 (March 1986), which gives the following definition:
"A ceramic is a polycrystalline inorganic material, showing a complex microstructure of grains and grain boundaries. This structure is defined during the preparation cycle which converts raw materials, which are most often powdery, into a dense material that is ideally free of pores and has properties derived from those of its grains as well as from its heterogeneity. The essential technological stage in the preparation of a ceramic is its sintering, which is the temperature-atmosphere-time cycle during which the grains initially put into contact with one another by shaping operations get bound together through the effect of various transport mechanisms, and then acquire the desired microstructure".
It is known that, in the field of industrial ceramics, used notably in electronics, very special importance is attached to the development of powders derived by chemical process. These powders have definite advantages over the powders obtained by reaction in the solid state. These advantages are increased homogeneity and purity, easy control over the stoichiometry, high reactivity increasing the reaction kinetics and reducing the sintering temperatures (entailing in the elimination of lengthy, pollution-causing crushing operations), and the obtaining of a narrow grain size distribution enabling the growth of the grains to be controlled. However, apart from the drawbacks specific to these methods (namely, a large number of steps, very complicated chemistry for the formation of the precursors, a difficult calcination cycle--it sometimes implies the reduction of the specific surface area in a manner that substantially leads to fairly smooth grains that are often hard to sinter--, the presence of agglomerates with varying degrees of solidness depending on the powder drying and washing techniques), there is one drawback that concerns the relatively high cost of these preparation techniques.
In the field of capacitors, current trends, for nearly ten years, has been going towards the making of ceramic capacitors with high volume capacities (their development is closely linked to that of microelectronics) and towards the reduction of the cost of this type of component.
One approach envisaged consists in the use of dielectric materials, generally based on barium titanate, derived by chemical processes. These materials have improved dielectric properties (in particular, the increase in the dielectric constant owing to the purity of the precursors and the possibility of centering the distribution in size at an optimum value) and get sintered at lower temperatures. However, the often high cost price of powders derived by chemical methods has not yet permitted industrial-scale development.
In the field of high frequency applications (microwaves), the dielectric resonators that are made should have a dielectric constant of more than 30, low losses (with a quality factor of more than 10,000) and a resonance frequency temperature coefficient of some ppm/.degree.C. These conditions of use very often call for the making of complex dielectric compounds with perovskite structure. Chemical methods are probably indispensable to obtain the perfect homogeneity of these ceramics.
Besides, it is known that positive temperature coefficient resistors (PTC thermistors) are made with barium titanate and that the amplitude of the resistivity jump is improved by the use of purer basic materials and by a more homogeneous distribution of the different dopants. The basic price of these components does not presently allow the use of the barium titanates prepared by liquid process owing to their excessively high cost.
The use of "soft" chemisty techniques to fabricate perovskite structure materials is not a new idea, and several methods are known. One known method is the coprecipitation method. In particular, the formation of barium and titanium double citrates or oxalates by precipitation of aqueous solutions Ti-Ba by the addition of oxalic or citric acid. This technique is described in W. S. Clabaugh, E. M. Swiggard and R. Gilchrist, "Preparation of Barium Titanyl Oxalate Tetrahydrate for Conversion to Barium Titanate of High Purity", J. Research N. B. S. 56, 5, 289 (1956) and in the U.S. Pat. No. 3,231,328. This technique has several drawbacks:
i. The operations for filtering and washing the precipitate as well as the unwanted precipitation of TiO.sub.2 make it difficult to control the stoichiometry.
ii. A calcination step is needed to decompose the double salt.
iii. The decomposition of the double salt does not lead directly to BaTiO.sub.3, because of the formation of carbonated intermediate products during the combustion of oxalate or citrate ions. Consequently, the sintering temperature remains high (1350.degree. C).
iv. The preparation of complex compounds by controlled precipation of a large number of elements is difficult.
The sol-gel technique is also known, notably from the U.S. Pat. No. 3,647,364. This technique entails the making of an amorphous gel by hydrolysis-condensation of a mixture of barium and titanium alkoxides in alcohol solution. For example, barium isopropoxide Ba(C.sub.3 H.sub.7 O).sub.2 and titanium butoxide Ti(C.sub.4 H.sub.9 O).sub.4 are hydrolyzed by an excess of water. The amorphous gel crystallizes at low temperature (500.degree. C) into very fine particles (5-10 nm). The following are the drawbacks of this technique:
i. The ultrafine powder, with a large specific surface area (50-70 m.sup.2 /g) easily forms agglomerates, and its shaping requires a preliminary calcination.
ii The very high reactivity to air of the barium alkoxides (hydrolysis) and of the amorphous gel (carbonation) requires implementation in inert atmosphere.
There is yet another known technique using barium hydroxide (see, inter alia, U.S. Pat. No. 3,292,994). The preparation of barium titanate, crystallized by the addition of a titanium alkoxide (for example Ti(O Pr.sub.4) to an aqueous solution of a barium salt, with a pH factor ranging from 11 to 14, has been reported by S. S. Flashen (J. Am. Chem. Soc. 77(1955)6194).
The main reactions that come into play during the process are: EQU Ti(OR).sub.4 +Ba.sup.2+ +2OH.sup.- +4HO.sub.2 .fwdarw.Ti(OH).sub.6.sup.2- +Ba.sup.2+ +4ROH [1] EQU Ti(OH).sub.6.sup.2- +Ba.sup.2+ .fwdarw.Ba Ti O.sub.3 +3H.sub.2 O[2]
In following the procedure described by Flashen, particles of BaTiO.sub.3 with a size of 1-5 .mu.m are obtained. The problem with this method is that of achieving the reactions completely. In effect, the formation of BaTiO.sub.3 leads to a gradual lowering of the concentration of the elements Ba and Ti in the solution and to a drop in the pH. Consequently, the reaction kinetics are gradually reduced, the reactions 1 and 2 are not total and the stoichiometry Ba/Ti is smaller than 1 and not reproducible. Several modifications have been proposed to improve this method:
The use of an excess of barium. The powder is then very sensitive to carbonation.
Hydrothermal synthesis (see, in particular, DE 3 26 674). Under conditions of 120.degree.-300.degree. C. and 0.5-5 MPa, BaTiO.sub.3, SrTiO.sub.3, and PbTiO.sub.3 powders have been prepared. Apart from the complexity of the method, the reactions do not seem to be complete, and the stoichiometry cannot be controlled.
To overcome these drawbacks, the invention proposes a simple method of fabrication, without the use of an inert atmosphere giving, within a wide range of compositions, ceramic powders that are crystallized after drying and have stoichiometry that can be controlled. The specific surface area of 10 to 20 m.sup.2 /g enables direct shaping without prior calcination. The powders can be sintered at low temperature (1150.degree. C. for BaTiO.sub.3). The simplicity of the equipment used, as well as the low cost of the raw materials, may make it easy to develop this technique on an industrial scale. The method is derived from the technique proposed by Flashen for the synthesis of BaTiO.sub.3. It consists in using an alkaline-earth hydroxide (Ba-Sr) hydrated in solid state, in dissolving it in an alcohol medium containing alkoxides. The dissolving is quickly followed by the reactions of hydrolysis-condensation of the alkoxides by the hydration water of the initial hydroxide, and the liquid mixture gradually changes towards the state of a homogeneous paste. The high concentration of the elements and the OH.sup.- ions in the liquid as well as the strong shaking that is kept up throughout the operation make it possible to achieve complete reactions and obtain a crystallized perovskite composition with controlled stoichiometry.